1. Field of the Invention
The invention relates to electrochemical reference cells, and more particularly, to systems incorporating such reference cells with improved liquid junctions for use in continuous monitoring of process streams.
2. Prior Art
Electrochemical reference cells used in pH or other specific ion meters typically employ calomel or other metal-metal salt reference electrodes immersed in a suitable electrolyte of known concentration that communicates through a liquid junction with the sample fluid being monitored. The liquid junction maintains a conductive bridge between the reference cell electrolyte and the liquid sample to provide a common potential in both solutions, while mixing of the two solutions is restricted to avoid changes in the electrolyte concentration that would vary the reference cell potential.
With earlier pH meters used for laboratory testing, the liquid junction was simply a minute opening in a glass or ceramic barrier through which ion communication between the two solutions could be established. However, with prolonged usage, the single opening junctions could readily be clogged. As a result, larger liquid junctions were developed using porous ceramic barriers, asbestos wicks, ground glass joints, or in some cases, an area of thin cracks produced by fusing together two glass formulations with different coefficients of expansion. However, to minimize mixing of the reference cell electrolyte with the sample fluid with these larger junctions, two or more liquid junctions placed in series were often required so that the reference cell electrolyte communicated through an intermediate salt bridge with the sample liquid. Nevertheless problems were still encountered in maintaining the liquid junction open for continuous process control use. Even with the larger, liquid junctions the small openings would eventually be plugged either by solid impurities in the process stream or by crystals formed within the reference cell where an electrolyte or saturated salt solution was used.
As disclosed in U.S. Pat. No. 3,440,525, issued to Charles P. Cardeiro, this problem was alleviated by the use of relatively large diameter wooden or porous ceramic plugs that established the liquid junction through minute capillaries extending longitudinally between the sample and reference fluids. The wooden plug was most effective and could be used wherever the cellulose structure was not dissolved or otherwise incompatible with the chemical solutions on either side of the junction. The numerous individual capillaries extending over the entire surface of the plug were not all readily clogged even under the worse sample process stream conditions, whereas the conductive bridge between the two liquids was maintained through the end walls of adjacent capillary cells the basic dilemma remained. Eventually, either the plug surface became completely covered with solid deposits or the salt ion exchange through the cell walls depleted the electrolyte concentration in the reference cell causing meter drift. Moreover, although the plug surface area could be increased to retard fouling by impurities, the resulting higher rate of ion exchange through additional capillary paths would more rapidly deplete the electrolyte concentration.